Can a single cable modem that has one outlet for USB and one for Ethernet be connected to two computers?

No you will have to use one or the other. In order to use more devices you will have to install a router to your modem.
It depends on the modem, some will let you use both ports simultaneously, while others won't. If you do have a modem that will run both, your ISP (cable/dsl company) will usually charge you a fee between $3.00 to $10.00 for accessing more then one dynamic/static IP, one for each port on the modem. So again your best bet is to get a router.
Connect one to the the Net then use windows internet sharing system ISS to share the link betwen the two systems. Use a wireless 802.3 connection or a second NIC on the ISS system to the cable modem and then connect the USB computer and "bridge" or ISS the two sub nets together witout a router!

Yes, I try this on my server & client PC's is this possible.when we connect one modem on two PC at same time it work, but when we connect two interface at one PC of separated two lan how can configure that?
It is possible to use two default getway on single PC?